Intrathymic injection in adult rats with guinea pig MBP or the immunodominant, encepalitogenic fragment p68-86 of MBP, in aqueous form -- without compromising the peripheral lymphocyte pool -- dramatically inhibits onset of EAE caused by vaccination with MBP in CFA. This finding demonstrates that interaction between aqueous antigen and one or more components of an intact thymus can regulate systemic responses by T-cells already existing in the peripheral lymphocyte pool. The goal of the studies proposed in this application is to develop a better understanding of the mechanism(s) involved in suppression of autoimmune disease by induced thymic tolerance, with a view of the possibility that this might be a useful approach for clinical treatment of autoimmunity. Using both the rat and mouse models for EAE, the cell subsets involved in thymic tolerance will be explored: Whether thymic tolerance can be transferred adoptively with thymus cell suspensions, whether cytokine profiles are altered, whether the more mature thymocyte population is involved, whether thymic antigen presenting cells (APCs) inhibit T-cell responses; and whether inhibitory thymic cells traffic to the CNS. Specificity studies will explore two aspects of thymic tolerance: Whether ongoing immune responses to multiple antigen systems can be regulated by induced thymic tolerance to one of them, and whether TCR and/or MHC binding residues of the tolerizing peptide are involved in thymic tolerance. These studies relate to the question of whether thymic tolerance is akin to other mechanisms of peptide induced immune hyporeactivity. Cytokine profiles of peripheral lymphoid cells from thymic tolerant animals before and after culture with antigen will be explored. Adaptive transfer experiments will explore the significance of a population of TCR+ CD8+ and TCR+ CD4-8- T cells that appear, along with dramatically increased levels of IL-4 mRNA, in antigen selected cultures.